Tunnel kiln



NDV. 15, 193s. T. G. MCDQU'GAL TUNNEL KILN Filed Jan. 4, 1936 5 Sheets-Sheet l Elmw Nov. 15, 1938. 1". G. McDoUGAL TUNNEL KILN I 5 sheets-sheet 2 vFiled Jan. 4, 1936 l T. G. McDouGAL Nav. 15, l193s.v

TUNNEL KILN File'duan. 4, 19:56,

5 Sheets-Sheet 3 Nov. 15, 1938. T. G. MCDOUAL TUNNEL KILN Filed Jan. 4, 1936 5 sheets-sheet 4 Patented Nv.1s,1a3s i 2,131,091' TUNNEL KILN Taine G. McDougal,Flint,I Mich., assignor to General Motors Corporation, Detroit, Mich., a cor- H'poration of `Delaware Application January 4, 1936, Serial No. 57,499

13 claims. (cms-142) This invention has to do with improvements in methods of ring ceramic ware to high temperatures, and in the Aconstruction of vkilns in which the tiring is done. The invention isv especially applicable to kilns 'of the continuous or 'tunnel type such as are disclosed and claimed in my prior Patents No. 1,416,726, granted May 23, 1922, No. 1,713,851, granted May 21, 1929, and No. 1,713,852, granted May 21, 1929, respectively.

The principal object of the invention is to so improve methods of firing ceramic ware as to obtain higher firing temperatures with a minimum of burdenon the refractories and conveyor parts and with a high degree of eiiiciency. This 15 is accomplished by projecting flame directly upon the ware so that it is exposed to the'maximum temperature available while the refractories and other parts are heated as by convection and radiation and somewhat by theremaning portions of the flame.`

-'Io accomplish this in a tunnel kiln the burners in the heating zone are arranged so that burning the air-fuel mixture impinges directly on the articles being red. With this disposition it is l5 possible to obtain temperatures at the surface of the ware substantially higher than the temperatures ofthe refractory walls of the kiln, thereby increasing the life of these refractories. Especially where maturing temperatures of the order of :.0 1800 C. are desirable, the disposition herein disclosed will make such temperatures commercially attainable where otherwise their use would be substantially impracticable.

Another important feature of one variation of the kiln structure consists in the provision of a double wall for the top of the kiln of which the lower wall carries no load, save its own 1eight, but serves as a reector for radiating heat onto the bodies being preheated. The lower wall or -10 arch is provided with iiues for conducting combustion gases from the hot zone to the chamber above it, which gases together with those passing underneath maintain the lower top wall at desirably higher temperatures in the preheating 45 zone. The nues permit readyescape of burnt gases from the firing chamber, thereby facilitating combustion, and making it possible to reduce,

the size of the combustion chamber without correspondingly cutting down the temperatures 50 attained.

It is also preferable to extend the double walled roof construction throughout all, or at least a substantialmortion of the preheating zone, thereby Aproviding 'a flue Ileading exhaust gases to a Il stack adjacent the entrance of the kiln. 1n order to recover some of the heat contained in the burn gases, it is preferred to provide pipes in a por- -tion ofthe passage to carry air on its way to the burners, thereby raising its temperature.

Other features of the'invention consist in the means for cooling the cars carrying the warel through the tunnel as well as the tracks on which they run and the supports therefor. OtherA details of the invention will be discussed in the,

course of the following description.

^ In the drawings:

Figures 1, 1a and'lb constitute together a-longitudinal section through `an improved kiln embodying the invention.y

Figure 1c is a continuation of Figure 1b showing the cooling zone in side elevation.

Figure 2 is a section on line`22 of Figure 1 showing in addition the support for the kiln.

Figure 3 is a section an line 3-3 of Figure la.

Figures 4, 5 and 6 are sections on corresponding` of thev so-called miniature type described and claimed ln my prior Patent No. 1,713,851. 'I'hls type of kiln is designed tol ilre ware passed through it preferably in singlele. vIt possesses the advantage of requiring but a short time,'on

the order of two hours, for the passage of ware through the kiln, and' its capacity is'substantially large even when compared with the much larger cross-sectional kilns which necessitate on the order of 36to 72 hours for firing the ware.

The kiln consists of the preheating zone which comprises all of Figures l and lav and the portion marked Preheating zone in Figure 1b; the heating zone shown at the center of Figure 1b, and the cooling zone shown at the right of Figure 1b and in Figure 1c.,

'I'he kiln is of comparatively small size, being designed especially -for the ring of spark plug insulators, and for convenience in operation and repair it has been found desirable to mount it porting members canies at its top a member l2,

adapted to carry the opposed base plates I 4 spaced apart to receive the vtrack I6. -The track IE- may Abe raised or loweredv by adjusting the supporting it to a suitable pump located at the entrance end of the kiln, the pump supplying the air to pipe 24 which conducts combustion air to the burners.

It will be noted that the track I6 is of U- shape, and it is provided with a fall on the order of one inch in its entire length so v that water may be fed in at one end, and discharged from the other to assist in cooling the track and the Cars.

" so that the air is heated by the flue gases.

pipe 2 4 is preferably supported on spaced blocks A Ihe sides of the track member I6 are grooved as shown at 26 to receive the flanged` wheels 28 of the cars 30. The wheels 26 are mounted on axles journaled in car frame 32. The frames,

axles and wheels are preferably made of heat/ which, in turn, support refractories 38 carrying pins 40 on which the insulators 42 are mounted. To facilitate the insertion and removal of the pins 40 the refractories 36 are preferably provided with transverse openings 44 as shown.

'I'he heating zone of the kiln is shown in Figures 1b and 4. Here the kiln is formed to provide two tunnels 46 and 48, one above the other. The walls of the tunnels are lined with high temperature refractory blocks, and the upper Wall 50 of the tunnel 48 is in the formof an arch which supports the weight of the refractories above it. The upper wall 52 of the tunnel 46 carries its own weight only, and is provided with spaced flues 54 connecting the tunnels.

The tunnel 48 acts asa flue extending from the heating zone throughout the length of the preheating zone to the stack 50.

The pipe 24 conducting air to the burners 52 extends throughout-the major portion of the length'of the tunnel 48 in the preheating zone The 55 made of suitable refractory material. The pipe 24 leaves the preheating zone at some suitable point, such as indicated at 56 in Figure 1a, and then extends along the top of the kiln, preferably embedded in the loose refractory material there provided, and conducts air to suitable manifolds 56 where it is distributed to the burners 52 as shown in'Figure 4.

The burners 52 may be ofany suitable type and are preferably fed with gaseous fuel. Suitable vanes may be provided to give a whirling motion to the entering fuel or air so as to insure thorough mixture. The combustible mixture is projected under`pressure through a suitable orifice 60, preferably provided with a restriction or venturi, directly onto the ware 42 so that a very high temperature is maintained at Ithe surface of the ware. No doubt surface combustion takes place on the wares surface. A portion of the combustion gases passes through the ilues 54 into the upper tunnel 48.

' With this construction of kiln` ithas been found possible to obtain temperatures as much as 60 to 150 C. higher at the surface of the ware than at the walls of the tunnel so that the refractories are subjected toless burden, while at the same time the necessary higher temperatures are attained at the ware.. At high iiringV temperatures, for example, around 1750 C., the problem of providing suitable refractories for the walls of the kiln is a diflicult one, and by this method of firing therefractory problem is capable of practical solution at reasonable cost.

The burners 52 are preferably arranged in staggered groups as shown in Figures 1b and 4, a group on one side of the kiln beingfollowed by a group on the opposite wall ofthe kiln so as to apply the intense heat to both sides of the in sulator.

The preheating zone is characterized by the factthat the tunnel 46 extends throughout its entire length as does also the tunnel 48, except that the entrance end of ,tunnel 48 is closed. Flues 66 connect the tunnels at spaced points. The flue 66 nearest the entrance is of considerable size so as to afford the burnt gases passing through the tunnel 46 ready access to the stack 50.

direct cooling zonemade of solid masonry as shown in Figure 5; next, an air-jacketed masonry cooling zone as shown in Figure 6; and, preferably spaced from the end of the latter, a direct air cooling zone shown at the extreme right of Figure lb and in Figure 1c. In the last named zone air is supplied through the conduit 68 to pipes 'i0 inclined, toward the entering ware, and the pipes 'I0 direct the air through the sheet metal tunnel portions l2, preferably spaced from each other and from the'end of the kiln proper as shown. The air under pressure travels through the tunnel l2 in a direction opposite to the direction of feed of the Ware so as to offset the tendency of the combustion gases to pass outwardly through the coling zone.- At the same time the pressure should not be suiiicient toforce air in toward the heating zone as this would tend` to reduce uneconomically the temperature in the hot zone. l

In the operation of the kiln, loaded cars are introduced at the left of Figure 1, and are continuously fed through the preheating zone where the temperature is gradually raised by the direct action of flue gases traveling toward the stack through tunnel 46 and by the heat radiated from the 'wall 52 which is heated by the flue gases traveling through both tunnels 46 and 48. The flue gases traveling through tunnel 48 raise the temperature of the-combustion air in pipes 24 to quite a high degree, preferably on the order of 400 C., and this air is delivered under pressure to the burners where it mixes with the fuel, preferably ordinary commercial gas, or butane. To insure adequate preheating of the ware, as Well as of the combustion air it may be desirable to provide baiiies here, and there in tunnels 46 and/or 48 to control the velocity of the flue gases.

The thoroughly heated bodies then pass into the heating zone where they are subject to direct impingement of the flame from the burners, thus providing the lmaximum temperature of combustion at the surface of the ware. This heating is preferably effected. alternately, rst on one side, and then on the other, but, if desired, burners could be arranged in opposed relation to heat the two sides simultaneously. The iiues 54 provide adequate scavenging insuring the highest volumetric efliciency in the combustion chamber. Actual installation temperatures as high as 1750o C. have been obtained at the face of the ware, while the temperatures at the sur-V The @001mg zone 1s divided into, nrst, an inl walls of the kiln were on the order of 150 lower. The fired wares then pass into the cooling zone where they are successively cooled by the surrounding refractories of the indirect cooling zone, then by the cooling of the air-jacketed zone, and

inally by the direct air blast in the tunnel por,- tion 12. In some cases it may be desirable to use a water cooling zone between the air-jacketed portion of the tunnel and the direct air cooling zone, but in practice this Ahas not been found necessary.

While the ware has thus been subjected to gradual heating, intense heating 'and gradual cooling, the cars, and particularly the lubricated parts,such as the journals for the wheels, have been maintained at a temperature to insure good lubrication by the cooling system consisting of water owing through the track IGVand through the abutting water-jacketed portionsof the bas plates I4.

'I'he time of the ilring cycle may be very short. Thus, for maximum production Awhen firing at temperatures on theorder of 1750 C., the total time may beas little as one hour and twenty minutes.

The principal features of the invention as so far described may be applied as well to tunnel kilns in which the insulators are arranged in rowsrcrosswise of the cars. In Figure 9 thereis shown a vcross-section through the hot zone of such a kiln. The principal change consists'in the provision of burners 80 in the upper'wall of the kiln, -these burners projecting the'ilameY directly upon the insulators,just as do the burners 8| in the side'walls. In this design of kiln it may be found desirable to employ staggered banks of burners along the side of the kiln as in the form nrst described.

In the kiln shown in Figure 9 the double tun- 40 nel construction hasy not been employedal-` ame upon the ware, said last-named means being focused on the ware so that substantially all of the flame impinges on the ware, producing combustion at the surface thereof.

2. In the combination as defined in claim 1, said firing zone being provided with a tunnel 00 above thetunnel through which the wares are transported, said tunnels having a common wall, said wall being provided with passages for conducting combustion gases fromthe mst-named tunnel to the second-named tunnel.

65 3. In the combination as defined in claim 1, said iring zone being provided with a tunnel above the tunnel throughv which the wares are transported, said tunnels having a common wall, said wall being provided with passages at the side I0 of the tunnels for leading combustion gases froml the rst-named tunnel to the second-named tunnel.

. 4. In a tunnel kiln a ring zone comprising a j tunnel, means for continuously transporting ware through the tunnel, means in the firing zone of the tunnel for heating the ware by direct impingement of llame, a second tunnel above the first-named tunnel, the top o fthe rst tunnel j forming 'the bottom of v the `second tunnel, and 6 nues connecting the tunnels whereby the combustion gases more effectively heat the [top wall ofthekiln. 1" l' 5. In the combination as defined in claim 4, said nues being located at the sides of the tunnels. l' 10 6. In a tunnel kiln having a tunnel including a preheating zone, a firing zone and a cooling zone,

i means for continuously passing ware through the tunnel, means for heating the ware in the iiring zone by direct impingement of name, a second 15 tunnel above the flrst'tunnel in the preheating and fixing zones, said tunnels having a common wall, and nues connecting said tunnels in the firing zone.

'1. In the combination as donned in claim 6, 20 conduits leading to said heating means arranged in said second-named tunnel for conducting heated air to the heating means.

8. In sftunnel kiln a ilring zone comprising a tunnel, means for continuously transporting 25 ware in single. ille through the tunnel comprising ware supporting means adapted to expose theware on substantially all sides, means on oppiosite sides of the tunnel for projecting fiameon the exposed sides of theware, said means being 3,0 focused Aon the ware so that substantially all of the ilarne impinges on the ware .producing combustion at the surface thereof.

a second Atunnel above the first-named tunnel,

the top ortho mst tunne1 forming the bottom of the second tunnel, flues connecting the tunnels at the sides thereof and arranged adjacent the flame projecting means, and conduits for leading 46 jair to the llame projecting means extending 'through the second-named tunnel.

11. A high temperature tunnel kiln for firing ceramic ware comprising a firing tunnel and an upper tunnel. said tunnels being separated by a 50 comparatively thin vheat transmitting and radiating partition, burners arranged in the firing tunnel so as to project llame on the ware therein producing maximum temperatures at the surface of the ware. and ilues connecting said timnels ad- 65 jacent the burners to afford ready escape for the products of combustion.

12 A tunnel kiln for tiring ceramicvware comprising upper and lower tunnels in theV Bring zone separated by a heat transmitting and radiating partition, bui-ners in the lower tunnel arranged to project name directly on the ware so as to produce maximum temperatures at the surface of the' ware, and ports adjacent the burners through which combustion gases in the firing zone canpassfrom'the lower tunnel to the upper tunnel.

13. A tunnel kiln according to claim 12 having a preheating zone comprising ai lower tunnel for the e offwares. and an upper tunnel, with ports connecting said tunnels through which combustion gases can pass from the lower tunnel.

TAINE G. McDOUG'AL. 

